Humankind has now spent an entire decade exploring interstellar space.
On Aug. 25, 2012, NASA's Voyager 1 spacecraft was 11 billion miles (18 billion kilometers) from the sun and scientists determined that the venerable spacecraft had crossed the boundary between the sun's influence and the interstellar medium. Now, still going and still sending back data, Voyager 1 and its twin Voyager 2 — which joined it in interstellar space in 2018 — continue to conduct ground-breaking science.
Launched in 1977, Voyager 1 and Voyager 2 were primarily designed to explore Jupiter and Saturn. Voyager 2 also visited Uranus and Neptune, while Voyager 1 instead prioritized Saturn's giant moon Titan, which sent it on a new trajectory to the edge of the solar system. It wasn't until after Voyager 2's flyby of Neptune in 1989 that Voyager's interstellar mission was created.
"Nobody ever expected the Voyagers to leave the interstellar medium," Merav Opher, an astronomer at Boston University who runs a center called SHIELD that's revisiting Voyager data in an effort to learn more about the heliosphere, told Space.com. "They had a five-year lifetime that was extended for another five years, then 10 years, but nobody really thought that they would be leaving the solar system."
What the twin spacecraft have discovered about the heliosphere — the magnetic bubble that surrounds us — could have consequences for our understanding of life on Earth.
Voyager 1's interstellar moment
Voyager 1 became the first human-made object to explore interstellar space — the space between the stars in a galaxy — when it was 122 astronomical units (AU) away from the sun. (One AU is the average distance between Earth and the sun and equals about 93 million miles or 150 million kilometers.)
Scientists know for sure that the Voyager probes have crossed from the heliosphere — where the solar wind, a stream of charged particles from the sun, dominates — into interstellar space because of particle density.
"The density was 10 times larger than what we find in the solar wind at those distances," Opher said of readings from late 2018. Voyager 1 was out.
Voyager 2's instruments detected the same jump in particle density when it entered interstellar space on Nov. 5, 2018.
Revealing the heliopause
The probes' crossing into interstellar space may have been clearcut, but the heliopause — a "twilight zone" between the bubble-like heliosphere and interstellar space — had unexpected properties. Each spacecraft took a couple of months to cross through, during which time they sent back data on peaks and troughs in plasma density.
"We expected it to be a bleep, but the heliopause is not like a line or a door — it's much thicker and more complex than we thought," Opher said. "But it's also a boundary that allows communication." The amazing discovery that particles come in and out of the heliosphere was from magnetic field data first from Voyager 1 in 2012, then from Voyager 2 six years later.
A magnetic mystery
Voyager 1 and 2's data on magnetic fields in the heliopause confused scientists. There ought to have been a change where the magnetic field from the sun met that from space, with theorists expecting the magnetic field from the galaxy to be inclined to the solar magnetic field. But neither Voyager 1 nor Voyager 2 detected magnetic field direction changes.
"When Voyager crossed there was no change in angle. The magnetic field stayed almost solar-like, with no rotation," Opher said. It's still a puzzle, but Opher and her collaborators have a theory about magnetic field flux tubes in the heliopause that connect the solar field with interstellar field. "It's almost like there are these highways for particles to go in and out of the heliosphere," she said, adding that it's probably a region where the magnetic field reconnects.
The heliopause appears to be the warped surface of the heliosphere that responds to solar activity, but why remains an open question.
Beyond the heliopause
Voyager 1 is now 40 AU beyond the heliopause, enabling scientists to discover what the interstellar medium is really like. It turns out to be much more influenced by the heliosphere than previously thought. "The interstellar medium as measured by the Voyager probes is not quiet, it's agitated and influenced by the sun," Opher said. "It's so different than we expected and we still don't really understand what's going on."
The data show that galactic cosmic rays behave differently depending on whether they're parallel or perpendicular to the sun's magnetic field. "We thought we'd see galactic cosmic rays coming from all directions, but they don't," Opher said.
The data can be correlated to coronal mass ejections (CMEs) from the sun that cause disturbances in its magnetic field far out into the interstellar medium. It's forced scientists to rethink how cosmic rays can reach the Earth.
What's next for the Voyagers
The "silent ambassadors" will, eventually, go quiet as they enter what scientists call the pristine interstellar medium. As the sun's influence wanes and there's less turbulence, the probes will likely pick up a matter-mix from other stars.
Detecting the influence of the next star along, however, is beyond the spacecraft. Voyager 1 will close in on a star in the Camelopardalis constellation called AC+79 3888 in 40,000 years while Voyager 2 is about the same flight-time from a star called Ross 248 in the constellation of Andromeda.
With the two spacecraft now closing out their operational lifetimes as their power supplies run dry, the last time scientists hear from them will probably be in the 2030s at best, mission personnel have said.
For scientists studying the heliosphere, the next spacecraft of interest will be New Horizons, the spacecraft that imaged Pluto in 2015. New Horizons will enter the heliopause around 2030, but its power supply won't last beyond the late 2030s. Meanwhile, scientists are considering Interstellar probe, a decades-long mission to study the heliosphere to build on the Voyagers' findings.
Voyager and the search for life
"There is nothing like in-situ data to reveal a new world and Voyager was instrumental in redefining what we understand about the heliosphere," said Opher, who thinks the heliosphere has shielded life on Earth from hazardous cosmic rays and dust. She also thinks that the solar system's position in the interstellar medium has been crucially important for fostering life.
"If we are going to try to find life on other planets then the only habitable astrosphere we have a handle on is our own," Opher said. "It was Voyager that revealed its complexity."